Durable Functions is an extension for Azure Functions to help write stateful services in a stateless environment. There’s an orchestration pattern named Fan out/fan in that can be helpful for contacting a number of external services and waiting for their replies. When one or more of these services throws an exception (for instance, calling EnsureSuccessStatusCode() on the response) it can be difficult to determine which service or services are having problems.

See the following:

var serviceUrl = "https://example.com";

var httpClient = httpClientFactory.CreateClient();
var response = await httpClient.GetAsync(serviceUrl);

// Example: "Response status code does not indicate success: 500 (Internal Server Error)."
// can be difficult to debug since it doesn't indicate the url for the service.
response.EnsureSuccessStatusCode();

Instead of EnsureSuccessStatusCode() change it to the code below:

var serviceUrl = "https://example.com";

var httpClient = httpClientFactory.CreateClient();
var response = await httpClient.GetAsync(serviceUrl);

if (!response.IsSuccessStatusCode)
{
    // Example: "GET https://example.com returned non-success HTTP status code: 500 InternalServerError."
    throw new HttpRequestException(
        $"{request.Method} {request.RequestUri} returned non-success HTTP status code: {(int)response.StatusCode} {response.StatusCode}.");
}

This can help identify the service or services that are having an issue when debugging issues with Durable Functions.